Due to the crucial role that rhythmic behaviors play in human interaction with the environment, the neural networks that generate rhythmic motor output have been extensively investigated. These networks were named central pattern generators (CPGs) because they can generate rhythmic motor outputs even in the absence of patterned sensory inputs of any type. The complex synaptic interconnectivity and active biophysical properties of CPG neurons endow these circuits with the ability to generate a multitude of motor patterns that match the behavioral output to the situational demands of the environment. It has been hypothesized, but not proven, that the selection of specific motor patterns is implemented by the action of neuromodulators that are released in response to specific configurations of stimuli. The aim of the research proposed in this application is to test this hypothesis in a simple model system in which specific modulator-containing neurons can be identified and studied while the behavior of the organism is monitored. A combination of electrophysiological, biochemical, immunocytological, and behavioral techniques will be applied to provide a well-integrated set of answers.